1. Field: The invention relates to the electrowinning of metal values from electrolyte solutions in electrolytic cells and to the construction of such cells.
2. State of the Art: In the electrowinning of metal values from solutions carrying such values, e.g. copper, nickel, or lead, the metal to be won deposits on a cathode starter sheet, and the quality of the deposition, both in chemical composition and physical shape, is of great concern. It is desirable to obtain a metal deposition of sufficient quality that it may be marketed directly to the consumer of the metal or used for other purposes without further refining and processing.
In an electrowinning cell, uneven current distribution upon the surface of the anode results in a corresponding uneven current distribution upon the surface of the cathode which causes the metal to deposit upon the cathode in non-uniform thickness and irregular formations. The irregular formations, or "mushroom-like growths" as they are sometimes called, cause serious problems in handling and subsequent processing of the metal cathode.
The formations usually appear along the edges and loops of the cathode, where the current distribution is high compared to other electro-active surfaces of the cathode where the current distribution is low and uniform. Since the rate of metal deposition is much greater in areas of high current distribution, the formations appear and develop into large masses at a higher rate of deposition than the rate of metal deposition on low current distribution areas. The formations increase so rapidly in size that, soon after the start of electrowinning, they touch the anode and cause extensive electrical shorting between the anode and cathode, reducing the current efficiency of the electrolytic cell and eventually stopping electrowinning altogether. Thus, the marketable product, namely, the metal cathode, is limited in thickness proportionate to the amount of time of the electrowinning. To deposit all of the metal values in a typical leach or solvent extraction electrolyte, the cathode starting sheets must be replaced several times.
In electrowinning, impurities in the electrolyte either remain in solution or settle to the bottom of the electrolytic cell in the form of a mud. The undesirable metal formations, which are deposited at the cathode, trap significant quantities of these impurities, typically lead and sulfur, electrolyte, stray solvent-extraction organic materials, and slime. As a result of these trapped materials, the electrowon metal is not of sufficient quality for direct sale to the consumer, for semi-continuous cake casting, for rod casting, or for many other applications. In addition, the electrowon metal products are very difficult to stack, store, and transport, as they bundle poorly because of the differences in thickness caused by the formations. Thus, the electrowon metal product must undergo further refining and processing before final usefulness is achieved.
The only way to eliminate development of these undesirable formations is to achieve a uniform current distribution on the surface area of the cathode. Heretofore, uniform current distribution has been achieved on the front and back surfaces and on the side and bottom edges by cutting the anode slightly shorter and narrower than the cathode. Normally electrical current flowing into the anode will be unevenly distributed, with higher current distribution on the suspension lugs and on the top, side, and bottom edges of the anode than on the front and back surfaces. When the anode and cathode are of the same dimension, the electrical current flux from anode to cathode induces a corresponding uneven current distribution on the cathode. When the anode is slightly shorter and narrower than the cathode, the high density electrical flux from the side and bottom edges of the anode redistributes itself on the greater surface area of the cathode such that the current distribution on the front and back surfaces and on side and bottom edges of the cathode is even.
Particular types of electrowinning processes require particular types of electrodes. Often the material from which the anode and cathode must be made and the amount of electrical current that the electrodes must carry restrict the size and shape of the electrodes, particularly the top portions thereof which must include suspension members for hanging the electrode from a hanger bar. Such suspension members must be sufficiently conductive and strong to support the main body of the electrode. Because of these size and shape restrictions, uniform current distribution cannot always be obtained along top areas of the cathode. This is true also because frequently the anode must be larger than the cathode in these upper areas.
No practical solutions for the problem of obtaining a uniform current distribution in such areas of the cathode are available in the prior art. In electrode pairs where this situation occurs, producers of electrowon metal have to settle either for metal depositions with undesirable extraneous formations on the top edge or for undesirably thin depositions.